Gas compression systems for methane, propane and the like, typically consist of two or more gas compressors with the gas flow between each pair cooled by an intercooler, and one or more power turbines which drive the gas compressors through a common drive shaft. A power turbine typically consists of a combustion chamber, turbine and an air compressor which provides compressed air to the combustion chamber. As is well known in the power turbine art, power increases as input air density increases, and air density increases as its temperature decreases. Further, the thermodynamic efficiency increases as inlet air temperature decreases. In addition, as a gas is compressed its temperature increases. Therefore, intercoolers are typically used to cool the gas between compressions stages thereby reducing the power required to further compress the gas in the next stage.
The present invention enables the energy-efficiency of intercooling to be increased. More specifically, the cooling requirements of a gas compression system is efficiently satisfied with an indirect evaporative film heat exchanger having separated dry and wet sides. In one embodiment, the heat exchanger provides a cool dry air stream and a cool moist air stream, respectively, to the power turbine and intercooler of the gas compression system described above. Although the specific type of indirect evaporative film heat exchanger is not critical to the present invention, it must be of the type that separately maintains air cooled on the dry side and air cooled on the wet side. Dry and wet side surfaces are formed by heat conducting walls so that air is cooled on the dry side without moisture absorption by thermal conductive contact with the dry side surface and air is cooled on the wet side by evaporation of water from the wet side surface. Cool dry air from the dry side and cool moist air from the wet side are separately exhausted from the heat exchanger to the power turbine air compressor and intercooler, respectively. Embodiments of this type of heat exchanger are described in our U.S. Patent application Ser. No. 601,873, filed Aug. 4, 1975 and entitled "EVAPORATIVE REFRIGERATION SYSTEM," now U.S. Pat. No. 4,023,949. It will be appreciated that this type of heat exchanger differs from a typical evaporative cooler in which all cooled air has been moistened by direct contact with evaporating water.
A gas compression system is especially suited to utilize the two types of cool air outputs of an indirect evaporating film heat exchanger. The cool moist air is used in the intercooler to cool the gas as it flows between compression stages and then discharged into the atmosphere. The cool dry air is supplied to the turbine air compressor. Thus the invention uses an indirect evaporating film heat exchanger having separated dry and wet sides to supply cooling air to a gas compression system consisting of a power turbine with its associated air compressor and combustion chamber, common drive shaft, gas compressors, and an intercooler interconnecting each pair of gas compressors. Air flowing over the heat exchanger dry side is sensibly cooled, i.e. without the addition of water vapor, and, in an embodiment, is provided to the power turbine air compressor. Air flowing over the heat exchanger wet side is cooled by evaporation therein, thus increasing its moisture content, and is provided to the intercooler to cool the compressed gas as it flows from one compression stage to the next.
A second embodiment provides even cooler air for the turbine air compressor and intercooler by connecting first and second indirect evaporating film heat exchangers in series so that the cool dry air output of the first heat exchanger is utilized as input air to the second heat exchanger. The very cool dry air output of the second heat exchanger is used as input air to the turbine air compressor. The very cool moist air output of the second heat exchanger is combined with the cool moist air output of the first heat exchanger and supplied to the intercooler.
A third embodiment utilizes the entire cool air output of an indirect evaporating film heat exchanger as the input air for a power turbine air compressor. In this embodiment cool dry air and cool moist air from the heat exchanger are combined prior to entering the air compressor, thereby increasing power turbine efficiency and power due to a decrease in inlet air temperature and increase in input air density. In another version of this embodiment cool dry air exiting from the dry side of the heat exchanger passes into the wet side, thereby causing the evaporation of water to take place at the depressed wet bulb temperature of the air. The very cool moist air from the heat exchanger is then directed to the power turbine compressor. It is essential that the moist air used as input air for the air compressor be free from water droplets containing dissolved salts or the like acquired within the heat exchanger. However, moist airstreams from some indirect evaporating film heat exchangers are not free of dissolved salts. Such heat exchangers at least partially accomplish wet side cooling by discharging water droplets into a moving airstream. Evaporation from the water droplet surfaces cools the moving air, but the salts remaining from complete evaporation of some droplets and the salt in partially evaporated droplets are carried along from the heat exchanger as suspended matter by the cool moist air. Evaporation of the droplets within the power turbine ducting creates a salt residue build up which could lower overall turbine life and efficiency. Therefore the heat exchanger used in this embodiment must output cool moist air substantially free from water droplets containing dissolved salts.
Cool moist air substantially free from salt containing water droplets can be obtained with a heat exchanger of certain configuration, modifying our prior heat exchanger of aforenoted application Ser. No. 601,873. In our prior heat exchanger walls are configured as vertically disposed, hollow elongated tubular conduits supported by top and bottom headers and the inner and outer walls of the conduit form the heat exchanger wet side and dry side surfaces, respectively. In modification of our prior heat exchanger, an insert is disposed within the top entrance aperture of each conduit, formed to ensure the discharge of a film of water of substantially uniform thickness over the inner wall of each conduit. The film of water flows downwardly over the inner conduit surface and evaporates in air flowed countercurrently upwardly through the circuit thereby creating cool moist air. The dissolved salts within the water are flushed downwardly by the flowing water. The evaporation from a film of water in flowing air rather than from water droplets suspended in the airstream prevents dissolved salts from contaminating the heat exchanger cool moist air output and damaging the power turbine by salt residue build-up.
Use of an indirect evaporating film heat exchanger having separated dry and wet sides to provide cool air inputs to a gas compression system increases its efficiency by lowering the temperature of the input air to the power turbine and providing cooling between gas compression stages. In this regard, one can refer to the following U.S. Pat. Nos.: 2,186,706, 2,766,886, 2,322,717, and 2,362,714. However, none of these patents disclose the use of an indirect evaporating film heat exchanger to provide cool air inputs to a gas compression system as hereinabove described.